Herds of reindeer and horses migrated across its plains, huge forests covered much of the countryside and men and women made their homes by rivers and lakes.

Then came the deluge, and this ancient Arcadia - which stretched across the North Sea, and covered the Channel - was inundated. All signs of human and animal activity were covered by several hundred feet of water. Only the occasional stone tool, bone harpoon and mammoth tusk, trawled from the sea bed by fishing boats, has provided reminders of this lost world's existence.

But the drowned lands of the North Sea and Channel may soon be revealed by British scientists using a revolutionary underwater scanning technique that can create sea-bed maps and images as accurate and detailed as those made of dry land. In the process, the idea of Britain as an island kingdom will be challenged by researchers.

'For the first time, we have the technology to map the North Sea and Channel sea beds in unsurpassed detail,' said Dr David Miles, chief archaeologist of English Heritage. 'That offers us a unique chance to open up our history. There could be dozens of perfectly preserved sites down there.'

Miles is to meet senior government officials in the next few days to press for their backing for a project which would use techniques developed by researchers at Imperial College, London to survey the sea bed inside British territorial waters. The team, led by Dr Sanjeev Gupta, used an advanced sonar technology, known as bathymetry, to study the sea floor several miles off the coast of Sussex.

Ten thousand years ago, at the end of the last ice age, with much of the world's water still trapped in giant glaciers and ice caps, sea levels were many feet below their current level. The river Arun, which now enters the Channel at Littlehampton, then ran on for several miles before flowing into a valley that had been carved by a south-flowing river created by the combined waters of the Thames, Rhine and Seine.

Only a vague outline of the extended Arun river bed had been made until Gupta and his colleagues - who presented the first results of their surveys at a archaeology workshop this month - began their mapping project. They used a special echo sounder, strapped to the hull of their survey boat, to make great sweeps across the sea bed. Previously, echo sounders have only been able to plot in two dimensions, but by using computers, satellite positioning devices and special software, their bathymetry device was able to compensate for sea swell and ship movement and create a perfect three-dimensional image of the landscape beneath their craft.

'What we saw was astonishing,' added Gupta. 'The topography was incredibly detailed, rich and complex. We could see the river bed that the Arun had created thousands of years ago and examine the bays and cliffs along its valley. We could see a rocky ledge that might have formed a waterfall, an ideal place for an ancient settlement. We could also see where boats have since sunk and settled on top of this landscape.'

Now English Heritage, the agency that controls the nation's ancient monuments and which has also recently taken charge of maritime archaeology in Britain, is preparing to turn the techniques developed at Imperial College to create a national survey of Britain's territorial waters (which extend 12 miles from the coast), as well as mapping regions of the North Sea. To do that, they will need government and industry funding.

'Most of the world's seabed is still a mystery to mankind,' said Miles. 'We have better images of Mars and Venus than of two-thirds of our own planet. Now we can put that right. And given that Britain is a maritime nation, for whom the sea has been an immense influence, finding out its secrets around our shores is particularly important. For example, we know there was once a Roman lighthouse near Whitby. We could pinpoint that with this technique.'

Researchers are especially keen to create seabed maps so they can identify places humans might have picked for their homes and villages: sheltered bays, cliffs that would have contained caves, and freshwater lakes. The most promising candidates could then be selected for special attention. Divers could be sent down, robot submersibles used to investigate and samples dredged from surface boats. The aim would be to find stone tools, house timbers (which survive far longer in water sediments than on dry land) and other evidence of human activity.

'Britain has been occupied by human beings for the past 500,000 years and during that time sea levels have dropped, linking our land to Europe on five occasions,' said Gupta. 'In fact, we were more often connected to the continent than we have been an island. Our current status is really the unusual one. That makes this sort of research all the more important.'

However, the use of bathymetry scanners will not be confined merely to studying lost landscapes and drowned settlements. It will also be invaluable to the pinpointing of lost wrecks, as Dr Ian Oxley, English Heritage's head of maritime archaeology, pointed out. 'According to national records, there are 44,000 known shipwrecks off our shores. But that is only a tiny sample, we know. By extrapolating from shipping figures, we believe the true figure is more like half a million. This technology will also be crucial in helping us find these.'

Scientists are keen to map the seabed as soon as possible because increased commercial activity - marine cable laying and the dredging of special Channel trenches for container ships - threatens not just the archaeology of this lost world, but its rare plants and shellfish beds. Key sites could be identified and then protected by law.

'We have a good relation with industry over this,' added Miles. 'The money for the bathymetry project was provided by the new aggregates levy that companies have to pay to minimise their impact on the environment, and they recognise the importance of this sort of surveying as well. There are immense practical benefits for them - pinpointing new underwater aggregate sites, for example. There is something for everyone.'